Why uranium dioxide (UO2) is preferred over metallic uranium as reactor fuel: which combination of material properties correctly explains the choice for most power-reactor fuel?

Introduction / Context:
Most commercial power reactors employ uranium dioxide (UO2) fuel pellets inside zirconium alloy cladding rather than metallic uranium. Material selection balances thermal, mechanical, chemical, and neutronic constraints to ensure reliable performance across long irradiation cycles.

Given Data / Assumptions:

• We compare bulk material properties relevant to reactor operation.
• Focus is on melting point, irradiation stability, chemical behavior, and density.
• General power-reactor context (e.g., PWR/BWR fuel).

Concept / Approach:
UO2 is a refractory ceramic with a very high melting point (~2800°C) and strong chemical stability under appropriate coolant chemistry. It resists radiation-induced swelling and phase changes better than metallic uranium, which has multiple allotropic forms and a much lower melting point (~1132°C). Although metallic uranium has higher density and thermal conductivity, its metallurgical complexities and lower melting point make it less suitable for standard light-water reactors.

Step-by-Step Solution:
Check (a): UO2 higher melting point than U metal → true.Check (b): UO2 does not have higher density than uranium metal; U metal is denser (~19 g/cm^3) than UO2 (~10.5–11 g/cm^3) → false.Check (c): As a ceramic, UO2 exhibits good irradiation stability and chemical compatibility in fuel–cladding–coolant systems → true.Therefore, the correct combined choice is “Both (a) and (c).”

Verification / Alternative check:
Standard fuel handbooks list UO2 pellet properties, including high melting point and acceptable irradiation performance; metallic uranium fuels were used historically but presented dimensional instability and metallurgical challenges.

Why Other Options Are Wrong:
(b) Incorrect: density comparison is the opposite; uranium metal is denser than UO2.“All (a), (b), and (c)” is invalid because (b) is false.

Common Pitfalls:
Equating higher density with universally better fuel without considering thermal margins and stability; assuming ceramics are inferior due to lower thermal conductivity while ignoring their high melting point and stability advantages.

Final Answer:
Both (a) and (c).